This is a 12 month study of omega-3 fatty acids in bipolar disorder. This study will be a 12-month, parallel group, double-blind comparison of the prophylactic efficacy of omega-3 fatty acids vs. placebo in 120 bipolar I patients. All subjects entering the primary prophylactic study will be euthymic or have only subsyndromal mood symptoms for at least 4 weeks. In addition, their concomitant medication (only lithium, divalproex, or no medication will be permitted) will also be stable and at accepted therapeutic levels for at least 4 weeks. An 8-week lead-in phase will be available to subjects who do not meet the current symptom and concomitant medication inclusion criteria (however, subjects must meet all of the other inclusion/exclusion criteria): 1. 4 weeks of euthymic or subsyndromal mood. 2. Subjects who are not already receiving lithium or divalproex. 3. Subjects receiving other psychotropic medications.